The principal objective is to study correlations between cell proliferation kinetics, rate of growth, degree of differentiation of transplantable mouse mammary carcinomas and their response to X- irradiation. Specifically, the proposed aims are: (a) to define morphologically and to quantitate the proliferating and nonproliferating cell population of tumors; (b) to evaluate epithelial tumor cells in respect to their degree of maturity and differentiation, and to establish their kinetic parameters; (c) to determine the selective response to irradiation of the proliferating and nonproliferating cell populations; (d) to study the effects of irradiation on proliferation kinetics of tumor cells differing morphologically and functionally; and (e) to correlate kinetic parameters with individual responses of different cell-types to irradiation. It is anticipated that these studies will provide basic information for further studies on recovery after irradiation. The relationship between proliferating and nonproliferating cell pools and the transition from a nonproliferating to a proliferating pool will be analyzed in tumors irradiated with fractionated doses of X-irradiation. The information obtained from these studies may serve as a basis for designing more effective therapy of solid tumors.